1. Field of the Invention
The present invention relates to an apparatus and a method for producing a magnet roller which is incorporated in a developing roller, a cleaning roller, a toner carrying roller, or the like used for an electrophotographic copier, a laser beam printer, a facsimile, or the like.
2. Description of the Related Art
An electrophotographic apparatus or an electrostatic recording apparatus such as a copier and a laser beam printer is designed to make a magnetic developing agent, namely, toner, which is carried by a developing roller, adhere to an electrostatic latent image formed on an image bearing member such as a photosensitive drum, thereby developing the image. The developing roller has a magnet roller which is formed using a resin-bonded magnet and which is disposed in a rotating nonmagnetic sleeve; it carries the magnetic toner to a vicinity of the surface of the photosensitive drum in such a manner that the magnetic toner is shaped like a spike or like a thin layer on the surface of the sleeve, then it develops an image by making the toner adhere to the electrostatic latent image by bringing the toner in contact with the photosensitive drum or by jumping development.
Hitherto, the aforesaid magnet roller is produced by shaping a pelletized resin magnet composition, which is composed of a thermoplastic resin binder such as nylon and polypropylene with magnetic powder such as ferrite mixed therein, by injection molding or extrusion molding using a metal mold with a magnetic field generator disposed around the metal mold, thereby magnetizing the roller to a desired magnetic characteristic. In this case, the roller is usually provided with shafts on both ends thereof; a single shaft may be fed through the roller in the axial direction to provide the shafts on both ends, separate shafts may be provided on both ends, or the shafts on both ends or a shaft on one end is molded integrally with a roller body by using a composition for resin-bonded magnet.
The manufacturing method for the magnet roller whereby the metal mold with the magnetic field generator disposed therearound for injection molding is called a magnetic field injection molding method. According to the magnetic field injection molding method, a melted resin-bonded magnet material composed primarily of resin powder and a binder is injected into the cavity of a two-piece metal mold, then a magnetic field is applied from outside around the cavity of the metal mold to magnetize it, thereby producing a resin magnet. Then, the metal mold is cooled to harden the magnet roller made of the resin magnet and the mold is split to take out the completed magnet roller.
Referring to FIG. 14, the manufacturing method for a magnet roller which is based on the conventional magnetic field injection molding method will be described. FIG. 14 is a cross-sectional view of a resin-bonded material which has been injected in a magnetic field injection molding metal mold 100 constituted by cover mold counterparts 101 and 102, dell pressure being applied thereto. A cavity 104 for the magnet roller is formed by the cover mold counterparts 101 and 102, approximately half thereof being in the respective cover mold counterparts. The mold temperature is maintained at a fixed level by a cooling pipe, not shown. One of the cover mold counterpart 101 or 102 is mounted, as a fixed mold, on the resin-bonded magnet material injection molding apparatus, not shown, and the other is mounted as a movable mold. A magnetic field generator 106 is provided in the vicinity of the metal mold 100. The injection molding apparatus is equipped with a nozzle 105.
As shown in FIG. 14, to mold the magnet roller by the magnetic field injection molding method, the metal mold 100 is formed by clamping the fixed mold counterpart and the movable mold counterpart with a parting line 107 as the boundary, a melted resin-bonded magnet material 10 is injected through an injection hole 102 A while applying a magnetic field to the cavity 104 of the metal mold 100 by the magnetic field generator 106, thereby magnetizing the resin-bonded magnet material 10 to mold the resin magnet. After cooling time elapses, the metal mold 100 is released and the magnet roller made of the resin magnet is taken out.
The magnet rollers manufactured according to the conventional magnetic field injection molding method, however, have such defects as flow marks on the surfaces thereof or bubbles at the central portions thereof from time to time.
FIG. 15 is a typical explanatory diagram showing the flow marks and bubbles that take place in injection moldings. These defects are likely to occur when a melted resin with high viscosity is used for injection molding. When the viscosity of the melted resin is high, melted resin 111 is shaped like a string in the early stage of filling when it is injected from the gate into a metal mold 110 as illustrated in FIG. 15(A). As is schematically shown in FIG. 15(B), the melted resin, which has been injected in the string-like shape, ends up being charged in the cavity in a compressed state. Melted resin 112, which has been shaped like entangled strings, is compressed and fused, leading to the formation of a fused mark 113 which is known as the flow mark. Further, as schematically illustrated in FIG. 15(A), when the string-like melted resin is injected into the metal mold, air in the cavity is sometimes caught in a spot 115 between the entangled melted resin strings. The result is a bubble 116 in the finished molding as shown by the cross section of FIG. 15(C). This is the defect known as the bubble inside.
Even if magnet rollers are free of such defects, they tend to have warp in the lengthwise direction more or less, whereas they should be straight in the axial direction. If a magnet roller is warped, then the gap between the surface of the roller and the sleeve, which is formed when a nonmagnetic sleeve is attached, varies according to the lengthwise position of the roller. As a result, the developing roller in which the warped magnet roller has been incorporated will have a magnetic force characteristic which varies in the axial direction of the roller, causing unstable toner carrying function. The magnetic pole of the warped magnet roller is frequently twisted in the lengthwise direction.
Thus, the magnet rollers manufactured according to the conventional method have many defects and incur warp more or less, leading to the need for inspection on every magnet roller for warp in the production process. This calls for inspection equipment and inspection man-hours; those magnet rollers, the warp of which exceeds a tolerance, are eliminated as defective products.